Implantable cardioverter defibrillator (ICD) systems are extremely reliable. However, the endocardial leads used in such systems tend to be the source of problems if they do occur. This is true because the leads are exposed to the hostile environment of the human body and are subjected to continuous flexure with each heart beat and crush at their introduction site. Further, the design requirement that the leads be as small in diameter as possible to fit through a vein results in a design which is somewhat vulnerable to mishandling. One of the most frequently used lead insulation materials is silicone rubber which has a tendency to propagate nicks or cuts. While polyurethane is less susceptible to cuts, it has shown a greater tendency to degrade with time. Both materials may fail when subjected to the phenomenon known as clavicular crush which is caused by pressure on the lead where it is positioned between the patient's clavicle and the underlying muscle. Another source of such problems is damage caused by sutures tied around leads for fixation of the leads. Thus, lead insulation failure has been one of the most frequent sources of problems with ICD systems, particularly with regard to the production of sensing artifacts.
In addition to problems with insulation, lead conductor fracture resulting in an intermittent make-break phenomenon can occur and can also cause sensing anomalies. This problem may typically result both from the constant flexing of the lead and clavicular crush mentioned above.
A further problem with sensing artifacts which has been identified results from fluid penetration in the connector port for the sensing conductor of an ICD. This can result in shorting between the two sensing conductors and sensing of artifacts by the ICD which are interpreted as ventricular fibrillation. See Hief et al., "Cardioverter Discharges Following Sensing of Electrical Artifact Due to Fluid Penetration in the Connector Port", PACE, August 1995, pp. 1589-1591.
A fracture in a lead conductor, a lead insulation failure which contacts one or more sensing conductors in the lead or fluid penetration in a connector port can cause artifacts to be sensed by the sensing and analysis circuitry of the pulse generator. These artifacts may contain frequencies which can be analyzed as tachycardia or fibrillation resulting in the initiation or delivery of one or more inappropriate high voltage shocks to the patient. It would be desirable to have an ICD system which is capable of determining whether a particular sensed electrogram is contaminated with sensing artifacts so that inappropriate shock delivery may be avoided.
It is thus an object of the invention to provide a method for determining whether an electrogram signal includes sensing artifacts caused by lead fracture, lead insulation failure or connector port fluid penetration.
It is another object of the invention to provide an ICD system which is capable of discriminating between an electrogram signal which includes sensing artifacts and one which does not.